Method of producing saturated C3-C20-alcohols

ABSTRACT

The invention relates to a process for the preparation of saturated C3-C20-alcohols in which a liquid hydrogenation feed comprising at least one C3-C20-aldehyde is passed over a bed of a hydrogenation catalyst in the presence of a hydrogen-containing gas, which comprises adding to the hydrogenation feed an amount, homogeneously soluble therein, of a salt-like base. The addition of base suppresses side reactions, such as acetalization, aldolization, Tischtschenko reaction or ether formation.

This application is a 371 of PCT/EP01/05676, filed May 5, 2001.

The present invention relates to a process for the preparation ofsaturated C₃-C₂₀-alcohols in which a liquid hydrogenation feedcomprising at least one C₃-C₂₀-aldehyde is passed over a bed of ahydrogenation catalyst in the presence of a hydrogen-containing gas.

The catalytic hydrogenation of aldehydes in order to obtain alcohols isa process which has been carried out on an industrial scale for decadesand in which a multiplicity of catalysts, which generally consist ofelements from sub-groups VI to VIII and I of the Periodic Table, inparticular of the elements chromium, manganese, iron, cobalt, nickeland/or copper, are employed. Such catalysts are described, for example,in DE-A 32 28 881, DE-A 26 28 987 and DE-A 24 45 303. The alcoholsprepared by these processes find broad use, for example as solvents oras plasticizer alcohols.

In the hydrogenation, in particular at high hydrogenation temperatures,undesired side reactions, such as acetalization or aldolization, theTischtschenko reaction or ether formation, occur in addition to thedesired hydrogenation of the aldehyde to the alcohol. These sidereactions result in a reduction in the product yield and requireincreased effort during purification of the hydrogenation product inorder to obtain the relevant alcohol in the desired purity.

In order to avoid side reactions of this type, DE-A 26 28 897 recommendsadding water to the hydrogenation feed. However, this measure has anumber of disadvantages; for example, the energy necessary forpurification of the resultant alcohols by distillation increasessignificantly.

Another possibility for reducing the formation of by-products comprisesincreasing the hydrogen pressure in the hydrogenation, which increasesthe rate of the hydrogenation reaction, while the reaction rate of theside reactions which are independent of the hydrogen pressure remainsthe same. Overall, the selectivity with respect to the desiredhydrogenation product thus increases.

However, an increase in the hydrogen pressure is associated with highequipment complexity, since, for safety reasons, pressurized reactorswith thicker walls must be used and further safety precautions have tobe taken.

German patent 16 43 856 describes the hydrogenation of aldehydes bymeans of a copper- and/or nickel-containing catalyst whose surface hasbeen adjusted to a pH of from 6 to 10 by treatment with alkali metalhydroxides. This publication is expressly directed to the use of thecatalysts pretreated in this way in gas-phase hydrogenation. Their usein liquid-phase hydrogenation is only possible to a limited extent. Thealkali metal hydroxide is usually washed out by the liquid hydrogenationfeed or the liquid hydrogenation products and removed from the reactionsystem, and consequently the advantages of the surface treatment of thecatalyst are only short term.

JP 172 838 A relates to the hydrogenation of C₅- and higher aldehydes ona nickel/chromium catalyst in the presence of a tertiary aliphaticamine.

JP 171 447 A relates to the hydrogenation of C₄-aldehydes to butanol ona nickel/chromium catalyst in the presence of a tertiary aliphaticamine. In both the last-mentioned processes, the added amine isseparated off from the hydrogenation product by subsequent distillationand advantageously fed back into the hydrogenation. However, pure amineis not recovered in the distillation, but instead a mixture of the aminewith so-called high boilers, i.e. the by-products which boil higher thanthe target alcohol and are formed in the hydrogenation of aldehydes, isobtained. The recycling of the amine/high boiler mixture requires that aballast of high boilers is always circulated through the hydrogenationand distillation. Since, in order to avoid increases in concentration, apart of the high-boilers which corresponds to the formation rate of thehigh boilers must always be removed from the circuit, amine losses areunavoidable and represent an additional economic burden for the process.

WO 96/26173 describes a process for the purification of C₃-C₁₀-alcoholsby distillation, where the distillation is carried out in the presenceof an alkali metal hydroxide. This publication makes no mention of theaddition of a salt-like base to a liquid hydrogenation feed.

It is an object of the present invention to indicate a process for thepreparation of saturated alcohols from aldehydes by liquid-phasehydrogenation in which the formation of undesired by-products issuppressed, in particular at hydrogenation temperatures of 150° C. orabove, and which is free from the disadvantages of the knownhydrogenation processes.

We have found that this object is achieved by a process in which aliquid hydrogenation feed comprising at least one C₃-C₂₀-aldehyde ispassed over a bed of a hydrogenation catalyst in the presence of ahydrogen-containing gas, which comprises adding to the hydrogenationfeed an amount, homogeneously soluble therein, of a salt-like base[M⁺]_(n) [A^(n−)], in which [M⁺] is an alkali metal ion or theequivalent of an alkali earth metal ion; [A^(n−)] is an anion of an acidhaving a pK_(s) value of greater than 2, and n is the valency of theanion.

The effect of the addition of base to the hydrogenation feed is that theside reactions outlined at the outset are substantially suppressed evenat hydrogenation temperatures of 150° C. or above, and very purealcohols are obtained even at these hydrogenation temperatures.

The type of salt-like base used is generally not crucial so long as thesalt-like base used is homogeneously soluble in the hydrogenation feed,at least in low concentration, and does not undergo any undesired sidereactions with the aldehyde. Accordingly, a multiplicity of salt-likebases can successfully be employed in the process according to theinvention.

The bases employed in accordance with the invention are salt-like, i.e.they are built up from cations and anions; they comprise at least onealkali metal or alkaline earth metal cation, such as lithium, sodium,potassium, magnesium or calcium ions, and a basic anion. Thecorresponding acid of the basic anion has a pK_(a) value of greater than2, preferably greater than 4, in particular greater than 8. The pK_(a)value used for the characterization of the acid strength of thecorresponding acid is the negative decimal logarithm of the dissociationconstant of the acid in dilute aqueous solution. The pK_(a) values ofnumerous acids have been tabulated and are given, for example, in CRCHandbook of Chemistry and Physics, 76^(th) Edn., 1995, CRC Press;Organikum, various authors, 16^(th) Edn., VEB Deutscher Verlag derWissenschaften 1986, p. 138; Sykes P., Reaktionsmechanismen der Org.Chemie, 8^(th) Edn. 1982, p. 307.

Suitable basic anions are hydroxide (14), carbonate (10.33),hydrogencarbonate (6.35), phosphate (12.35), amide (35), hydride (39);alkoxides, in particular C₁-C₄-alkoxides, such as methoxide (16),ethoxide, n- and isopropoxide and butoxide; phenoxide (10),carboxylates, such as acetate (4.76) or benzoate (4.21); carbanions,such as butyl (50), cyclopentadienyl or phenyl (40). The values inbrackets indicate the pK_(S) value of the respective corresponding acid.Besides the hydride ion itself, complex hydrides are also suitable;these can be regarded as adducts of the hydride ion and their basicityis essentially due to the, hydride ion, for example complex hydridessuch as [BH₄]⁻ or [BHR₃]⁻ (where R=C₁-C₄-alkyl, for example s-butyl).

In general, hydroxide or carbonate is preferred.

Advantageous salt-like bases are, in particular, alkali metal hydroxidesand/or carbonates, such as lithium carbonate, potassium carbonate,sodium carbonate, lithium hydroxide, sodium hydroxide and potassiumhydroxide. In general, sodium hydroxide and/or potassium hydroxide arepreferred. However, sodium alkoxides and/or potassium alkoxides, such asthe methoxide or ethoxide, or the alkoxide of the alcohol which is thehydrogenation product of the aldehyde present in the hydrogenation feedcan also be employed with particular advantage.

The salt-like bases are generally added to the hydrogenation feed in anamount which corresponds, on neutralization equivalent basis, to from0.1 to 2000 ppm by weight, preferably from 0.1 to 1000 ppm by weight, inparticular from 0.1 to 100 ppm by weight, particular preferably from 0.5to 50 ppm by weight and in particular from 1 to 20 ppm by weight, basedon the aldehyde present in the hydrogenation feed, of potassiumhydroxide. In the case of monovalent basic anions, a molar amount ofsalt-like base which corresponds to the stated amount of potassiumhydroxide is used, and in the case of divalent basic anions, half themolar amount is used. It is also possible to add mixtures of differentbases.

Owing to the low salt-like base concentrations employed and thegenerally low price of these bases, recovery is not necessary oradvantageous.

The liquid hydrogenation feed can consist of one or more undilutedaldehydes. However, the aldehydes are preferably employed as a solutionin an inert diluent. Examples of suitable inert diluents arehydrocarbons, ethers, such as diethyl ether, or alcohols. The diluentsare particularly preferably alcohols, in particular the alcohol which isthe hydrogenation product of the aldehyde to be hydrogenated. In apreferred embodiment, a part amount of the hydrogenation product isrecycled for this purpose and mixed with the aldehyde to behydrogenated. If used, the inert diluent is preferably used in an amountof from 0.1 to 100 parts by weight, in particular from 1 to 50 parts byweight and particularly preferably from 5 to 20 parts by weight, basedon one part by weight of aldehyde employed. If the hydrogenation iscarried out adiabatically, i.e. with removal of the heat of reaction bythe reaction product, the amount of inert diluent used is advantageouslysuch that the temperature gradient over the bed of the granular catalystdoes not exceed 40° C. If, by contrast, the hydrogenation reactor isoperated isothermally, the proportion of inert diluent in thehydrogenation feed can be selected virtually as desired.

The hydrogenation feed generally contains traces of water, for examplein the order of from 1 ppm to 1% by weight, which have been introducedby the starting materials in the preceding synthesis steps or formed bycondensation reactions. These traces of water are unimportant for theprocess according to the invention. On use of salt-like bases other thanhydroxides, hydroxide ions are formed therefrom by transprotonationand/or hydrolysis, which is in accordance with the invention.

The process according to the invention can be carried out eitherbatchwise or continuously, for example with the aid of tubular reactorsor reactor cascades. The catalyst bed generally rests on a suitableretention element in the reactor. The hydrogenation reactor can beoperated either by the pool or trickle method. The process according tothe invention is preferably carried out in a reactor cascade, inparticular a cascade comprising two to five reactors.

The salt-like base can be added in solid or dissolved form; it ispreferably added in the form of its solution in water or an alcohol, inparticular the alcohol which is the hydrogenation product of thealdehyde present in the hydrogenation feed. For example, from 1 to 40percent by weight solutions are suitable. The base and the hydrogenationfeed can be introduced into the hydrogenation reactor separately fromone another, with the mixture of base and hydrogenation feed forming insitu in the reactor. In particular in the case of the continuousprocedure, however, a preformed mixture of based and hydrogenation feedis preferably passed into the reactor. If, as in a preferred embodiment,a part amount of the hydrogenation product is fed back as diluent beforethe hydrogenation reactor, the metering of the base is advantageouslycarried out into the return stream before the latter is mixed with thealdehyde to be hydrogenated. In this way, local concentration maxima ofthe base during contact with the aldehyde, which can result in undesiredaldolization, are avoided. If a reactor cascade is used, the requisiteamount of base can be passed into the first reactor of the cascadetogether with the hydrogenation feed; however, it is also possible tometer the base separately into each individual reactor of the cascade.The total amount of base is preferably fed into the first reactor of thecascade together with the hydrogenation feed.

The hydrogenation catalyst used is one of the catalysts usually used forthe hydrogenation of aldehydes to alcohols. The type of catalyst used isnot a subject-matter of the present invention; the advantageous effectsachieved by the process according to the invention are generallyindependent of the type of hydrogenation catalyst used. Accordingly, amultiplicity of hydrogenation catalysts can be used in the processaccording to the invention, for example metal-containing supportedcatalysts with metals from sub-group(s) I, VII and/or VIII of thePeriodic Table as catalytically active components, in particularsupported catalysts with rhenium, platinum, palladium, rhodium and/orruthenium as catalytically active components and support materials suchas aluminum oxide, titanium dioxide, silicon dioxide, zirconium dioxideor barium sulfate; or precipitation catalysts comprising at least oneelement from sub-group(s) I, VI, VII and/or VIII of the Periodic Table,for example catalysts as described in DE-A 32 28 881, DE-A 26 28 987 andDE-A 24 45 303. The catalysts are preferably in granular form andgenerally have a particle size of from 3 to 10 mm. The catalysts may bearranged in one or more beds in the reactor. Different catalysts can beused in the various beds of a reactor or in the various reactors of areactor cascade.

The hydrogen-containing gas preferably comprises more than 80 mol % ofhydrogen; in particular, it essentially consists of hydrogen. Thehydrogen-containing gas can be passed over the hydrogenation catalystbed in cocurrent or countercurrent to the hydrogenation feed. It ispreferably passed in cocurrent. The amount of hydrogen-containing gasfed in is advantageously such that from 1.0 to 1.15 times thestoichiometrically necessary amount of hydrogen is available.

The hydrogenation of the alcohols can be carried out under conditionswhich are conventional per se. In general, elevated temperatures, forexample from 100 to 300° C., preferably from 120 to 250° C. and inparticular from 130 to 200° C., and pressures of from 1 to 700 bar,preferably from 5 to 300 bar and particularly preferably from 30 to 50bar, are set. The catalysts are generally loaded with from 0.01 to 2,preferably with from 0.1 to 1 and in particular with from 0.2 to 0.5 lof aldehyde/l of catalyst per hour. The addition of water to thehydrogenation feed is possible in the process according to theinvention, but is not necessary.

The alcohols are generally worked up by distillation by methods knownper se.

The aldehydes to be hydrogenated are preferably aliphatic C₃- to C₂₀-,in particular C₃- to C₁₅-aldehydes, which may be straight-chain orbranched and may additionally contain double bonds in the molecule. Thealdehydes which can be employed are not subject to any restrictions inprinciple. Suitable aldehydes which are of particular economicimportance are, for example, propanal, n-butanal, isobutyraldehyde,hexanal, ethylhexanal, ethylhexenal, nonenal, nonanal, decanal, decenaland the hydroformylation products of trimeric and tetrameric propyleneand dimeric and trimeric butene.

The process according to the invention has a number of advantages:

The acetal and ether formation side reactions which occur duringhydrogenation of aldehydes in the liquid phase are greatly suppressed.The addition of water to the hydrogenation feed which was customaryhitherto can be reduced or omitted completely. This enables the energyconsumption in the subsequent distillation of the hydrogenation productto be significantly reduced, since the water is generally distilled offat the top of the column. The hydrogenation temperature can be increasedwithout the fear of an increase in side reactions. This enables thespace-time yield to be increased; for example, an increase in thehydrogenation temperature from 140 to 150° C. in the hydrogenation ofbutanal allows the catalyst loading with butanal to be increased by 25%for the same butanol yield. An increase in ether formation on increasingthe hydrogenation temperature, which is to be expected without additionof base, is not observed. The increase in the hydrogenation temperatureliberates the heat of hydrogenation at a higher temperature level andcan be utilized for generating steam at 4 bar, for example in theintegrated heat system of the hydrogenation plant. This results in aconsiderable saving of energy.

The invention is illustrated in greater detail by the examples below.

EXAMPLES

The aldehydes were hydrogenated in a reactor cascade consisting of anadiabatically operated first reactor having a capacity of 1450 l and asecond reactor operated isothermally at 130° C. having a capacity of 225l.

The hydrogenation feed comprising n-butanal (obtained byhydroformylation of propene) and crude butanol was fed to the tworeactors in such a way that the liquid hourly space velocity was notless than 20 m² per hour. The crude butanol with which the butanal wasmixed in the hydrogenation feed had previously been taken off from thebottom of the first hydrogenation reactor by means of a circulationpump. The second reactor served as post-reactor in order fully tohydrogenate the butanol reacted only incompletely in the first reactor.The composition of the hydrogenation product was determined by gaschromatography before its distillation.

Example 1 Not According to the Invention

2900 kg of a modified Adkins catalyst (Lit.: J. Am. Chem. Soc. 51, 2430(1929); J. Am. Chem. Soc. 54, 4678 (1932)), which, in the unreducedstate, comprised 35% by weight of copper, calculated as Cu, 31% byweight of chromium, calculated as Cr, 2.0% by weight of barium,calculated as Ba, and 2.5% by weight of manganese, calculated as Mn,were subjected in the apparatus described above to a hydrogenation feedas indicated above which was composed of 325 parts by weight ofn-butanal and 5000 parts by weight of circulated crude butanol. Beforeuse, the Adkins catalyst had been reduced at 300° C. in a stream ofhydrogen until water was no longer formed. The hydrogen pressure in thereactor was 40 bar, the temperature on entry into the catalyst bed was103° C., and the temperature at the outlet of the first reactor was 132°C. The reactor was operated for 180 days in this manner. The crudebutanol obtained had the following composition (anhydrous):

n-butanol 98.85% by weight di-n-butyl ether  0.11% by weight butylbutyrate  0.12% by weight butyraldehyde di-n-butyl acetal  0.89% byweight

The carbonyl number in accordance with DIN 53 173 as a measure of theresidual aldehyde content was determined as being 0.1 g/g of crudebutanol. The carbonyl number is the amount of potassium hydroxide in mgwhich is equivalent to the amount of hydrogen chloride liberated fromhydroxylammonium chloride on oximation of 1 g of substance.

Example 2 According to the Invention

As described in Example 1, a hydrogenation feed comprising 430 parts byweight of n-butanal and 5000 parts by weight of crude butanol washydrogenated, with 3 ppm by weight of potassium hydroxide, based on then-butanal supplied, being added to the hydrogenation feed. Thetemperature at the catalyst bed inlet was 102° C., and the temperatureat the outlet was 137° C. The hydrogen pressure was 40 bar. The reactionproduct had the following composition (anhydrous):

butanal    0% by weight n-butanol 99.68% by weight di-n-butyl ether 0.01% by weight butyl butyrate  0.24% by weight butyraldehydedi-n-butyl acetal  0.07% by weight

The carbonyl number, as a measure of the residual aldehyde content ofthe crude butanol, was 0.1 mg/g of crude butanol. The carbonyl numberdid not worsen even after an operating time of 60 days.

Example 3 According to the Invention

In the apparatus described above, a catalyst in accordance with DE-A 2628 987, which, in the unreduced state, comprised 24% by weight ofnickel, calculated as NiO, 8% by weight of copper, calculated as CuO,2.0% by weight of manganese, calculated as MnO, from 66% by weight ofSiO₂ as support material, was introduced into the reactor in reducedform. For the reduction, the catalyst had been treated at 200° C. in astream of hydrogen until water no longer formed.

500 parts by weight of n-butanal, 15 parts by weight of water and 5000parts by weight of circulated crude butanol were introduced at the topof the reactor, and such an amount of a solution of potassium hydroxidein n-butanol was metered into this mixture that the feed comprised 7 ppmby weight of potassium hydroxide, based on the supplied butanal. Thepressure in the reactor was 38 bar, the temperature at the catalyst bedinlet was 126° C., and the temperature at the outlet from the firstreactor has risen to 150° C.

The hydrogenation product had the following composition (anhydrous)after an operating time of 142 days:

butanal  0.01% by weight n-butanol 99.41% by weight di-n-butyl ether 0.04% by weight butyl butyrate  0.02% by weight butyraldehydedi-n-butyl acetal  0.53% by weight

Example 4 Not According to the Invention

960 kg of a catalyst comprising 19.6% by weight of copper and 0.2% byweight of sodium on a support of silica gel beads having a diameter of3-6 mm were installed in the apparatus described above. The catalyst waspre-reduced.

A hydrogenation feed comprising 425 parts by weight of n-butanal and5000 parts by weight of circulated crude butanol was introduced at thetop of the 1^(st) reactor. The hydrogen pressure in the reactor was 36bar, the temperature at the catalyst bed inlet was 81° C., and thetemperature at the outlet from the first reactor was 115° C. The crudebutanol obtained had the following composition (anhydrous):

n-butanol 99.44% by weight di-n-butyl ether  0.01% by weight butylbutyrate  0.01% by weight butyraldehyde di-n-butyl acetal  0.44% byweight

The carbonyl number in accordance with DIN 53 173 as a measure of theresidual aldehyde content was determined as being 0.15 mg/g of crudebutanol.

Example 5 According to the Invention

As described in Example 4, a hydrogenation feed comprising 425 parts byweight of n-butanal and 5000 parts by weight of crude butanol washydrogenated, with 32 ppm by weight of potassium hydroxide, based on thesupplied n-butanal, being added to the hydrogenation feed. Thetemperature at the catalyst bed inlet was 81° C., and the temperature atthe outlet was 111° C. The crude butanol obtained had the followingcomposition (anhydrous):

n-butanol 99.81% by weight di-n-butyl ether    0% by weight butylbutyrate  0.04% by weight butyraldehyde di-n-butyl acetal  0.02% byweight

The carbonyl number, as a measure of the residual aldehyde content, was<0.01 mg/g of crude butanol.

Example 6a Not According to the Invention

As described in Example 4, 985 kg of a pre-reduced catalyst comprising24.1% by weight of copper and 0.27% by weight of sodium on a support ofsilica gel beads having a diameter of 3-6 mm were installed.

The catalyst was subjected, as indicated above, to a hydrogenation feedcomprising 650 parts by weight of n-butanal and 5000 parts by weight ofcirculated crude butanol.

The hydrogen pressure in the reactor was 36 bar, the maximum temperaturein the first catalyst bed was 126° C., and the maximum temperature inthe second reactor was 120° C. The reactor was operated in this way for120 days. The crude butanol obtained had the following composition(anhydrous):

n-butanol 99.12% by weight di-n-butyl ether  0.01% by weight butylbutyrate  0.02% by weight butyraldehyde di-n-butyl acetal  0.78% byweight

The carbonyl in accordance with DIN 53 173, as a measure of the residualaldehyde content, was determined as being 0.1 g/g of crude butanol.

Example 6b According to the Invention

The hydrogenation apparatus was operated further under identicalconditions as in Experiment 6a. 10 ppm by weight of potassium hydroxide,based on the supplied butanal, were added to the hydrogenation feed.Just 48 hours later, the product had the following composition:

n-butanol 99.82% by weight di-n-butyl ether    0% by weight butylbutyrate  0.05% by weight butyraldehyde di-n-butyl acetal  0.04% byweight

The carbonyl number, as a measure of the residual aldehyde content, was0.02 mg/g of crude butanol.

This example shows that the advantageous effects of the processaccording to the invention are also achieved in the case of catalystswhich are already in extended use and the effects arise rapidly.

Example 7

A hydrogenation catalyst in accordance with DE-A 26 28 987 having thefollowing composition was used:

24% by weight of nickel, calculated as NiO

8% by weight of copper, calculated as CuO

2.2% by weight of manganese, calculated as Mn₃O₄

0.6% by weight of sodium, calculated as Na₂O

remainder SiO₂

The hydrogenation was carried out in the apparatus described above underthe conditions indicated there. The two reactors were filled with atotal of 1600 l of the catalyst pre-reduced at 300° C. in a stream ofhydrogen.

The hydrogenation feed, which comprised from 360 to 475 kg of butanalmixture and 4200 kg of crude butanol, was fed to the two reactors insuch a way that the liquid loading per m² of reactor cross section areawas from 30 to 40 m³. The crude butanol had previously been taken fromthe hydrogenation product of the first reactor for the purposes ofrecycling. The temperature above the bed in the first, adiabaticallyoperated reactor was about 30° C.

The composition of the hydrogenation products were determined by gaschromatography before distillation.

Four experiments were carried out. In Experiments A and B, no base wasadded (not according to the invention). In Experiments C and D accordingto the invention, in each case 10 ppm by weight of potassium hydroxide,based on the aldehyde feed, were metered in. The results are shown inthe table below.

A: B: Starting materials: 370 kg of butanal 430 kg of butanal mixturemixture 4200 kg of crude 4200 kg of crude butanol butanol Temperature atthe 138° C. 144° C. inlet to the 1^(st) reactor: Pressure: 35 bar 35 bar

Composition of the hydrogenation product:

A: B: Butanals: 0.006% by weight 0.007% by weight Butanols: 99.2% byweight 99.44% by weight Di-n-butyl ether: 0.19% by weight 0.32% byweight Butyl butyrate: 0.03% by weight 0.03% by weight Butyraldehyde0.53% by weight 0.57% by weight di-butyl acetal: C: D: Startingmaterials: 430 kg of butanal 485 kg of butanal mixture mixture 4200 kgof crude 4200 kg of crude butanol butanol Base: 10 ppm by weight 10 ppmby weight of KOH of KOH Temperature at the 150° C. 150° C. inlet to the1^(st) reactor: Pressure: 35 bar 35 bar

Composition of the hydrogenation product:

C: D: Butanals: 0.01% by weight  0.02% by weight Butanols: 99.5% byweight 99.44% by weight Di-n-butyl ether: 0.01% by weight  0.02% byweight Butyl butyrate: 0.02% by weight  0.02% by weight Butyraldehyde0.46% by weight  0.50% by weight di-butyl acetal:

It follows from these results that the hydrogenation temperature andaccordingly the space-time yield can be increased with the aid of theaddition of base according to the invention without the increase in thehydrogenation temperature resulting in an increase in ether formation.

We claim:
 1. A process for the preparation of saturated alcohols inwhich a liquid hydrogenation feed comprising at least one aldehydeselected from propanal, n-butanal, isobutyraldehyde, hexanal,ethylhexanal, ethylhexenal, nonenal, nonanal, decanal, decenal and thehydroformylation products of trimeric propylene, tetrameric propylene,dimeric butene or trimeric butene, is passed over a bed of ahydrogenation catalyst in the presence of a hydrogen-containing gas,which comprises adding to the hydrogenation feed an amount,homogeneously soluble therein, of a salt-like base [M⁺]_(n) [A^(n−)], inwhich [M⁺] is an alkali metal ion or the equivalent of an alkali earthmetal ion; [A^(n−)] is an anion of an acid having a pK_(s) value ofgreater than 2, and n is the valency of the anion.
 2. A process asclaimed in claim 1, wherein the salt-like base used is an alkali metalhydroxide and/or an alkali metal carbonate.
 3. A process as claimed inclaim 1, wherein an amount of salt-like base which corresponds, onneutralization equivalent basis, to from 0.1 to 2000 ppm by weight,based on the aldehyde present in the hydrogenation feed, of potassiumhydroxide is added to the hydrogenation feed.
 4. A process as claimed inclaim 1, wherein the hydrogenation feed comprises an inert diluent.
 5. Aprocess as claimed in claim 4, wherein the inert diluent used is thealcohol which is the hydrogenation product of the aldehyde present inthe hydrogenation feed.